Power transmission shaft and method for producing same

ABSTRACT

The present invention provides a power transmission shaft that is capable of eliminating the usual tempering step thereby reducing the production cost, is capable of realizing an inline production process of the shaft, and has higher strength. The invention also provides a method for producing the power transmission shaft. The power transmission shaft is composed of a carbon steel having a carbon content of from 0.30 to 0.48 wt % and of a surface hardened layer  4  formed by high frequency quenching. The surface hardened layer  4  has a tempering-effected portion subjected to tempering using heat generated by external force applied after quenching. The residual stress of the portion subjected to tempering by applying external force is equal to or less than −800 MPa.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to power transmission shafts used for power transmission in automobiles and various kinds of industrial machinery, and more particularly, to a power transmission shaft used for constant velocity universal joints and a method for producing the power transmission shaft.

2. Description of Related Art

Power transmission shafts used for, for example, drive shafts and propeller shafts incorporated in the power transmission system of automobiles are usually composed of carbon steel used as the material of the shafts, and further are hardened on the surface thereof by heat treatment, thereby securing a predetermined strength. The power transmission shafts further include solid integral shafts and connection shafts obtained by welding or pressure welding.

In recent years, in response to an increase in vehicle weight due to enhanced output of automobiles or safety consciousness, there is a need for even higher strength for power transmission shafts. There is also an emerging need for lightweight power transmission shafts from the viewpoint of increasing fuel efficiency. In order to accomplish this need, the need for higher strength of the lightweight power transmission shafts becomes even more imminent. There are various suggestions in an attempt to make higher the strength of lightweight power transmission shafts (e.g., see patent documents 1 to 3).

A power transmission shaft disclosed in patent document 1 includes B added in a carbon steel, which is used as the material of the shaft, in an attempt to improve quenchability, reinforce grains, and reduce sensitivity against quenching cracking. The power transmission shaft also includes Nb, V, and Zr added therein in an attempt to improve toughness by crystal grain refinement.

A power transmission shaft disclosed in patent document 2 includes elements including Mo and B added in the steel material to form the base material structure into bainite and thus to extremely improve quenchability. Thus, patent document 2 attempts to improve quenching cracking resistance and fatigue strength by crystal grain refinement.

Thus, by adding special alloying elements having the effect of improving quenchability and of refining crystal grains, the power transmission shafts described in patent document 1 and patent document 2 attempt to make the strength of the shafts higher.

Incidentally, heat treatment of the power transmission shafts usually involves tempering after high frequency quenching and carburization quenching. In view of this, a power transmission shaft disclosed in patent document 3 adds shot peening after quenching and tempering in an attempt to make the strength of the shaft higher.

Patent document 1: Japanese Patent Application Publication No. 2000-234141. Patent document 2: Japanese Patent Application Publication No. 2005-060718. Patent document 3: Japanese Patent Application Publication No. 2003-307211.

However, adding a large amount of special alloying elements, as conventionally practiced, poses problems including an increase in the cost of the finished products. Also, forming the material structure into bainite in order to make the strength of the power transmission shaft higher is problematic in that increased material hardness lowers processability, which increases the cost of the finished products. Further, adding to the heat treatment shot peening after the usual quenching and tempering step results in a cost increase.

When B is added, as described in patent document 1, it is required to also add Ti in order to fully utilize the effect of adding B. This is problematic in that the TiN resulting from adding Ti forms a hard, large inclusion, which causes chipping during lathe turning, thus diminishing tool life. Further, improving quenchability by adding elements including Mo, as described in patent document 2, is problematic in that moldability is degraded. Also, the increase in S poses the problem of degraded jointability (molding/friction pressure welding properties).

The present invention has been accomplished in view of the foregoing problems, and it is an object of the present invention to provide a power transmission shaft that is capable of eliminating the usual tempering step thereby reducing the production cost, is capable of realizing an inline production process of the shaft, and has higher strength, and to provide a method for producing the power transmission shaft.

SUMMARY OF THE INVENTION

The power transmission shaft according to the present invention comprises: a carbon steel having a carbon content of from 0.30 to 0.48 wt %; and a surface hardened layer formed by high frequency quenching, wherein the surface hardened layer has a tempering-effected portion subjected to tempering using heat generated by external force applied after quenching.

Since the tempering step in the power transmission shaft of the present invention is carried out using heat generated by applying external force, there is no need for the usual tempering step. If the carbon content is less than 0.30 wt %, sufficient hardness and depth cannot be obtained after the quenching and tempering, causing the problem of degraded strength. If the carbon content exceeds 0.48 wt %, the moldability is significantly undermined, and further, machine processability is easily undermined and quenching cracking easily occurs.

The power transmission shaft preferably further comprises a notched portion subjected to tempering by applying external force. The tempering moderates the hardness of the shaft at the notched portion and thus increases the toughness characteristic thereof. The term notched portion, as used herein, refers to, for example, a base portion (spline tapered portion) of a spline portion formed at the end of the shaft.

The portion subjected to tempering by applying external force has a residual stress of equal to or less than −800 MPa. This provides an improved strength. If the residual stress exceeds −800 MPa, a sufficient strength (e.g., strength against the twisting fatigue test) cannot be obtained.

The method for producing a power transmission shaft according to the present invention comprises: subjecting a carbon steel having a carbon content of from 0.30 to 0.48 wt % to high frequency quenching; and subjecting the carbon steel to tempering using heat generated by applying external force.

Since in the method for producing a power transmission shaft according to the present invention the tempering is carried out using heat generated by applying external force, there is no need for the usual tempering step. Methods for applying external force include vanishing processing, hard turning, and shot peening.

Since in the power transmission shaft according to the present invention there is no need for the usual tempering step, the production cost can be reduced. Eliminating the tempering enables it to provide an inline production process of the shaft. Also, it is only the portion of external force application that obtains the tempering effect. That is, a power transmission shaft is easily obtained having the tempering effect only on a desired portion. Of particular notice is that the external force operates to apply a compressive residual stress, making it possible to provide a power transmission shaft having higher strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is front view of a power transmission shaft according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the power transmission shaft according to the present invention and the method for producing the shaft will be described in detail below.

FIG. 1 exemplifies, as an embodiment of the power transmission shaft, a case of the shaft being used for the drive shaft of an automobile. The drive shaft includes an intermediate shaft 1, which is the power transmission shaft, that is coupled with a sliding-type constant velocity universal joint at a shaft end portion 2 (not shown) by, for example, spline engagement and with a fixed-type constant velocity universal joint at the other shaft end portion 3 (not shown) by, for example, spline engagement.

On the end portions 2 and 3 of the shaft 1 are formed spline portions 5 and 6, respectively. The intermediate shaft 1 can be either a solid shaft processed from a solid bar material, a hollow shaft processed from, for example, a steel pipe, or a jointed shaft obtained by welding or friction pressure welding.

For the intermediate shaft 1, a carbon steel having a carbon content of from 0.30 to 0.48 wt % is used. Specifically, a material made of a carbon steel including from 0.30 to 0.48 wt % of carbon is heated with a high frequency, and heat resulting from the high frequency causes a hardened layer 4 to be formed on the surface of the material. The quenching temperature of the high-frequency heating is, for example, 1000° C.

The component of the material specified within the above range due to the following reasons. If the carbon content is less than 0.30 wt %, sufficient hardness and depth cannot be obtained after the quenching and tempering, causing the problem of degraded strength. If the carbon content exceeds 0.48 wt %, the moldability is significantly undermined, and further, machine processability is easily undermined and quenching cracking easily occurs.

The tempering of the power transmission shaft (intermediate shaft) 1 is carried out using heat generated by applying external force. Methods for applying external force include vanishing processing, hard turning, and shot peening. The term vanishing processing, as used herein, refers to the step of processing the cut surface of a processed object into mirror by pressing a vanishing roll onto the processed object using a predetermined pressing force. The term hard turning, as used herein, refers to processing providing a lathe finish using a super-hard tool, as an alternative of grinding carried out after the usual quenching. The term shot peening, as used herein, refers to a cold processing method such that hard microspheres called shot material are accelerated and sprayed from a projection device and allowed to have a fast collision with an object to be processed.

That is, the present invention enables it to apply external force to the power transmission shaft 1 at, for example, the notched portions, i.e., the base portions (spline tapered portions) of the spline portions 2 and 3 by the above-described external force applying means, without the usual tempering step. This causes the high-frequency quenched power transmission shaft 1 to generate heat, thereby being subjected to tempering. Thus, applying external force involves occurrence of a compressive residual stress, and the compressive residual stress is specified as equal to or less than −800 MPa. The residual stress can be varied depending on the temperature at which the residual stress occurs. If the residual stress exceeds −800 MPa, a sufficient strength (e.g., strength against the twisting fatigue test) cannot be obtained.

For the depth of the hardened layer, the ratio (t/r) of the effective hardening depth t to the radius r is equal to or higher than 0.40. If the ratio of the effective hardening depth to the radius is lower than 0.40, internally originating breakage occurs, causing the problem of significantly degraded strength.

Since in the present invention heat generated by applying external force is used, there is no need for the usual tempering step. This reduces the production cost. Eliminating the tempering enables it to provide an inline production process of the shaft. Also, it is only the portion of external force application that obtains the tempering effect. That is, a power transmission shaft is easily obtained having the tempering effect only on a desired portion. Of particular notice is that the external force operates to apply a compressive residual stress, making it possible to provide a power transmission shaft having higher strength. Specifying the carbon content between 0.30 wt % and 0.48 wt % provides sufficient hardness and depth after quenching and tempering, superior moldability, improves mechanical processability, and prevents quenching cracking.

Of particular notice is that the notched portions, which are the base portions of the spline portions 5 and 6, are subjected to tempering by applying external force, which is capable of moderating the hardness of the shaft at the notched portion and thus increasing the toughness characteristic thereof. Thus, the power transmission shaft has superior toughness.

Specifying the residual stress as equal to or less than −800 MPa provides an improvement in strength (e.g., strength against the twisting fatigue test).

While in this embodiment only the notched portions, which are the base portions of the spline portions 5 and 6, are rendered tempering-effected portions subjected to tempering by applying external force, the tempering-effected portions can be provided on other portions, and further, the tempering-effected portions can be provided all over the hardened layer 4 of the shaft.

EXAMPLE 1

As shown in Table 1, the inventive intermediate shaft 1 in the form shown in FIG. 1 and a conventional shaft in the same form as the intermediate shaft 1 were formed and subjected to a twisting test. As the shaft material both for the inventive and conventional shafts, SAE 1041 was used. Each material was subjected to high frequency quenching at a quenching temperature of 1000° C., after which the conventional shaft was subjected to tempering in a furnace for one hour at 180° C. The inventive shaft was subjected to shot peening in order to use the resulting heat for the tempering of the shaft. It is noted that the shot peening of the inventive shaft is not carried out after the usual quenching and tempering step, but carried out in place of the usual tempering step.

TABLE 1 Hardening Hardening ratio at ratio at spline flat Tempering Additional Residual Material portion portion condition processing processing Inventive SAE1041 0.4 0.6 — Shot −1000 MPa shaft peening Conventional SAE1041 0.4 0.6 180° C. —  −600 MPa shaft

As shown in Table 1, the inventive and conventional shafts have a hardening ratio (t/r) at the spline porions of 0.4 and a hardening ratio (t/r) at the flat portion of 0.6. The term flat portion, as used herein, refers to the middle portion of the intermediate shaft 1. The residual stress was −1000 MPa for the inventive shaft and −600 MPa for the conventional shaft.

Table 2 shows the results of a cyclic twisting fatigue strength test carried out with respect to the inventive and conventional shafts.

TABLE 2 Cyclic Twisting fatigue strength (1 × 10⁵ times) Inventive shaft 25% increase Conventional shaft Standard value

As shown in Table 2, the twisting fatigue strength of the inventive shaft increased by 25% relative to the twisting fatigue strength of the conventional shaft being rendered a standard. Thus, the present invention provides a significant improvement in strength. 

1. A power transmission shaft comprising: a carbon steel having a carbon content of from 0.30 to 0.48 wt %; and a surface hardened layer formed by high frequency quenching, wherein the surface hardened layer has a tempering-effected portion subjected to tempering using heat generated by external force applied after quenching.
 2. The power transmission shaft according to claim 1, further comprising a notched portion subjected to tempering by applying external force.
 3. The power transmission shaft according to claim 1, wherein the portion subjected to tempering by applying external force has a residual stress of equal to or less than −800 MPa.
 4. A method for producing a power transmission shaft comprising: subjecting a carbon steel having a carbon content of from 0.30 to 0.48 wt % to high frequency quenching; and subjecting the carbon steel to tempering using heat generated by applying external force after quenching.
 5. The power transmission shaft according to claim 2, wherein the portion subjected to tempering by applying external force has a residual stress of equal to or less than −800 MPa. 